The invention relates to a waveguide-type optical matrix switch which performs the switching and control operation of optical paths using optical waveguides provided on a substrate. The invention also relates to an optical ADM (optical add-drop multiplexer) having the function of performing optical-signal through, drop, and add.
In recent years, the advance of practical use of an optical communication system has led to a demand for an optical communication system having higher capacity and higher performance. In particular, in order to more stably and more efficiently operate many optical transmission lines, it has become necessary to properly recombine optical transmission paths upon occurrence of troubles of transmission paths and according to traffic. Further, also within an optical transmission apparatus, the recombination of optical paths within the apparatus upon occurrence of troubles of the optical device or the like has become necessary.
In order to cope with these demands, for example, there is a report on the use of an optical switch using a diffusion-type optical waveguide comprising an electrooptic crystal, typified by lithium niobate (LN), which has been mainly doped with titanium or the like through the surface of a substrate by thermal diffusion.
This optical switch is reported in xe2x80x9cDaikibo Doharokata Hikari Matorikkusu Suicchi (Large Waveguide Optical Matrix Switch)xe2x80x9d [Hideaki Okayama and Masato Kawahara, Singakugiho], TECHNICAL REPORT OF TEICE SSE 94-214, OCS 94-95 (1995-02), PP 67-72, or xe2x80x9cStudies on a 128-Line Photonic Space-Division Switching Network Using LiNbO3 Switch Matrices and Optical Amplifiersxe2x80x9d, (C. Burke, M. Fujiwara, M. Yamaguchi, H. Nishimoto, and H. Honmon, OSA Proceeding on photonic switching, 1991, Vol. 8, pp 2-6).
An optical matrix switch formed by integrating this type of optical switches is reported in xe2x80x9cPolarization Independent-DC Drift Free Ti:LiNbO3 4xc3x974 Matrix Optical Switchxe2x80x9d [Y. Nakabayashi, J. Ushioda, M. Kitamura, 2nd Optoelectronics Communications Conference (OECC""97) Technical Digest, July 1997, Seoul, KOREA, 9C5-3, pp 202-203].
There is also a report on an optical matrix switch which utilizes, for example, a change in refractive index of optical waveguides by thermooptic effect using a heater mounted on a part of quartz- or polymer-based optical waveguides. This optical switch is reported in xe2x80x9cDC-drift Free Polarization independent Ti:LiNbO38xc3x978 Optical Matrix Switchxe2x80x9d [Y. Nakabayashi, M. Kitamura, T. Sawano, 22nd European Conference on Optical Communication-ECOC""96, Oslo, ThD. 2.4.4, 157-4.160].
Here a single device having an optical path-switching function is called an xe2x80x9coptical switch,xe2x80x9d and a device or optical circuit, which can realize the switching of paths of a larger number of inputs and a larger number of outputs through a combination of a plurality of optical switches, is called an xe2x80x9coptical matrix switch.xe2x80x9d The way of combining optical switches within the optical matrix switch is called a xe2x80x9cnetwork.xe2x80x9d
In the optical matrix switch used in the switching of the conventional transmission path, reducing the level of crosstalk to approximately not more than xe2x88x9240 dB is required from the viewpoint of ensuring the transmission quality.
Also in the case where an optical matrix switch is used in the switching of path within the apparatus, minimizing the level of the crosstalk is desired. For most of the conventional optical switches reported up to now, however, meeting this requirement for low crosstalk level is difficult due to the performance of the optical switch per se.
Further, for example, in a multi-wavelength communication system using EDFA (erbium doped fiber amplifier), it is known that the optical power varies from a channel for one wavelength to a channel for another wavelength, for example, due to the dependency of the optical amplification factor of EDFA upon the wavelength and that this limits the transmission distance. Therefore, in a relay apparatus, for example, in a multi-wavelength communication system using EDFA, the function of eliminating the uneven optical power between the channels for respective wavelengths is necessary.
The optical matrix switch for the recombining light transmission paths is generally incorporated into a relay apparatus in a multi-wavelength communication system. For this reason, the optical matrix switch per se preferably has the function of, controlling the power of each optical channel from the viewpoint of reducing the size of the relay apparatus.
Accordingly, it is an object of the invention to solve the above problems of the prior art and to provide an optical matrix switch which can reduce crosstalk between channels and can, if necessary, control the power of optical channels.
FIG. 7 is a diagram showing the construction of a conventional optical ADM.
This optical ADM is provided along an optical transmission path having a plurality of channels (for example, 32 channels). An optical amplifier (AMP) 2001 is connected on the upstream side of the optical ADM, and a demultiplexer 2002 for demultiplexing a multiplexed optical signal to different wavelengths is connected to the optical amplifier 2001. 1xc3x972 optical switches 2003 having an identical construction are connected respectively to output lines of the demultiplexer 2002. For the optical switch 2003, one of the output terminals is a drop terminal, while one of input terminals in a 2xc3x971 optical switch 2004 is connected to the other output terminal of the optical switch 2003. The other input terminal in the 2xc3x971 optical switch 2004 is used as an add terminal. An attenuator (ATT) 2005 is connected to the output terminal of the 2xc3x971 optical switch 2004, and each input terminal of a multiplexer 2006 is connected to each output terminal of the attenuator 2005 in a 1:1 relationship. Further, an optical amplifier (AMP) 2007, which amplifies the multiplexed optical signal and output to the downstream side, is connected to the output terminal of the multiplexer 2006. Here optical devices, such as optical switches 2003, 2004, are connected to each other through an optical fiber. A photodetector (PD) 2008 is coupled to the optical fiber for connecting the attenuator 2005 to the multiplexer 2006. An automatic level controller (ALC) 2009 for controlling the attenuator 2005 is connected to the photodetector 2008.
The optical ADM shown in FIG. 7 is provided at a point C which is along an optical transmission line provided between points A and B distant from each other. A multiplexed optical signal from the point A is amplified in the optical amplifier to 2001, and is then demultiplexed in the demultiplexer 2002. According to the switching by the 1xc3x972 optical switch 2003, the demultiplexed signals are dropped at the point C (that is, is withdrawn to the outside of the system) or sent to the 2xc3x971 optical switch 2004 for transmission to the point B without drop. When the demultiplexed signals have been sent to the 2xc3x971 optical switch 2004, they are then sent to the attenuator 2005 through the 2xc3x971 optical switch 2004, where the control of the attenuation for output level matching is performed. The control of the attenuation in the attenuator 2005 is performed by controlling the gain or light transmission level of the attenuator 2005 through the automatic level controller 2009 based on a photoelectric conversion signal by the photodetector 2008. The optical signals from each of the attenuators 2005 are multiplexed in the multiplexer 2006, and the multiplexed light is amplified in the optical amplifier 2007 and sent toward the point B. When the 2xc3x971 optical switch 2004 is switched to the add side, optical information from the point C is input into the 2xc3x971 optical switch 2004 and is added to the multiplexed, optical signal from the point A. This type of ADM is described in detail, for example, in Masaki Fukui et al., xe2x80x9c1580 mn band all-optical and node prototype equipped with fast automatic level controlxe2x80x9d (24th European Conference on Optical Communication: 9.20-24, 1988).
According to the conventional optical ADM, since an optical fiber is used to connect optical devices to each other, a reduction in size, a reduction in weight, and a reduction in cost are difficult to realize. Further, there is crosstalk of the 1xc3x972 optical switch toward the non-output terminal. This makes it difficult to improve the extinction ratio. For example, even in the case of output to the drop terminal in the 1xc3x972 optical switch, input is performed through the terminal for through into the 2xc3x971 optical switch. Since, however, an attenuator is provided downstream of the 2xc3x971 optical switch, the above crosstalk cannot be prevented. Further, since an attenuator is provided at the terminal of the 2xc3x971 optical switch, the level control of the optical signal in the drop and add terminals is impossible. When a level controller is inserted in this site, the connection is carried out using an optical fiber. This renders the work troublesome, and leads to an increase in size. Accordingly, it is another object of the invention to provide an optical ADM which can realize a reduction in size, the suppression of crosstalk and the control of an optical signal level at the drop and add terminals without troublesome connection work and can easily realize an array construction and a multi-function.
According to the invention, an optical matrix switch comprises a branch-selective network in such a form that output ports of m (m=a positive integer) optical switches each with one input and n (n=a positive integer) outputs are connected to input ports of n optical switches each with m inputs and one output so that the n1st (n1=a positive integer) output port in output ports of the m1st (m1=a positive integer) optical switch with one input and n outputs is connected to the mist input port in input ports of the n1st optical switch with m inputs and one output, wherein the optical switches each with one input and n outputs and the optical switches each with m inputs and one output are formed of a material having electrooptic characteristics such that the refractive index changes upon the application of an electric field; and transmittance variable gate members are provided respectively between the output ports of the optical switches each with one input and n outputs and the input ports of the optical switches each with m inputs and one output.
That is, the optical matrix switch according to the invention comprises a branch-selective network and has, in its center stage, a transmittance-variable optical switch (a gate) with one input and one output.
At the output, the function of an optical switch having the simplest construction, that is, two inputs and two outputs (hereinafter referred to as xe2x80x9c2xc3x972xe2x80x9d), will be explained. In the case of the 2xc3x972 optical matrix switch, this network, when only the first and third stages are viewed, has the so-called xe2x80x9cTREExe2x80x9d construction, and a gate is inserted into the second stage as the center stage.
By virtue of this construction, crosstalk light can be attenuated by shutting off a gate disposed on a path in unuse. Further, the control of the transmittance of the gate permits the power of optical channels to be regulated according to need.
In general, in the case of mxc3x97n optical matrix switches, m switches each with 1xc3x97n are provided in the input stage, while n switches each with mxc3x971 are provided in the output stage. In this construction, the crosstalk light can be attenuated by providing mxc3x97n gates in the center stage and shutting off the gate disposed on a path in unuse. Further, the power of output from each channel can be regulated by controlling the transmittance of a suitable gate.
Thus, in a waveguide optical switch wherein the switching and control of optical paths are carried out using optical waveguides provided in the substrate, the crosstalk between channels can be reduced, and the power of optical channels can be regulated according to need.
Further, in order to attain the above another object, according to the invention, there is provided an optical ADM comprising: a substrate; a first optical switch which is provided on the substrate and outputs an optical signal, which has been input through a first input port, to either a first output port, (a terminal for drop) or a terminal for through; and a second optical switch which is provided on the substrate and outputs, to a second output port, either the optical signal output from the terminal for through in the first optical switch or an optical signal from a second input port (a terminal for add).
According to this construction, the first and second optical switches constituting the optical ADM and optical components connected to them are provided on a single substrate. Therefore, a reduction in size, a reduction in weight and a reduction in cost of the optical ADM can be realized. Further, increasing the number of switches according to the channels can easily realize an array construction and a multi-function on the substrate. This is suitable for the provision of multi-channel.
Further, in order to attain the above other object, according to the invention, an optical ADM comprises: a substrate; a first optical switch which is provided on the substrate and outputs an optical signal, which has been input through a first input port, to either a first output port (a terminal for drop) or a terminal for through; a second optical switch which is provided on the substrate and outputs, to a second output port, either the optical signal output from the terminal for through in the first optical switch or an optical signal from a second input port (a terminal for add); a first level controller which is provided on the substrate and controls the level of an optical signal directed from the first optical switch to the second optical switch; a second level controller which is provided on the substrate and controls the level of an optical signal output to the terminal for drop; and a third level controller which is provided on the substrate and controls the level of an optical signal input into the second-optical switch through the terminal for add.
According to this construction, a first level controller is provided between first and second optical switches for drop and add and controls the level of transmitted light, the level of an optical signal dropped is controlled in a second level controller, and the level of an optical signal added is controlled in a third level controller. The individual members are provided in an integral form on a single substrate. This can realize a reduction in size and weight of an optical ADM. Further, this enables the control of a variation in loss in each path and the control of the level of optical signals in a terminal for drop and a terminal for add without troublesome connection work. This can easily realize an array construction and a multi-function.